The present invention relates to an electrical signal filter with an improved isolation shield for magnetically isolating electrically interconnected filter sections from one another.
Various types of electrical signal filters are used in the CATV industry for controlling, on a frequency basis, the propagation of signals through a cable line. One example of such a filter is disclosed in U.S. Pat. No. 4,451,803, the entirety of which is incorporated herein by reference. The ""803 patent discloses a split tuning filter of the type that is commonly referred to as a notch filter, for removing a selected frequency or band of frequencies from a CATV signal. With reference to FIG. 7, the split tuning filter includes a common circuit board 100 having first 102 and second 103 filter sections formed thereon by discrete electronic components such as inductors, capacitors and the like (not shown). Isolation shields 104, 105 are arranged at a midpoint along circuit board 100 to provide magnetic isolation between first filter section 102 and second filter section 103. Each shield includes a radially extending disc section 106 and a longitudinally extending flange section 107. A slot 108 is formed in each shield, to allow the remaining, unslotted portion of disc 106 to slide into a corresponding slot 101 formed in circuit board 100. One of the shields is pressed into a slot formed on one side of the circuit board, and the other shield is pressed into a slot formed on an opposed side of the circuit board, as shown in FIG. 7. As explained in the ""803 patent, this arrangement prevents any xe2x80x9cline of sightxe2x80x9d communication between components in the first and second filter sections.
Once the shields 104, 105 are positioned on opposite sides of circuit board 100, the circuit board is inserted into housing 109, the open end of which is closed by filter cap 110. This subassembly is then inserted into a tube sleeve housing (not shown) to form the final filter structure.
While the filter disclosed in the ""803 patent is highly successful in providing magnetic isolation between the first 102 and second 103 filter sections, there are several drawbacks associated with the use of shields 104 and 105. First, the shields must be soldered not only to circuit board 100, but also to filter housing 109, in order to ground the circuit board. While the shields can be soldered to circuit board 100 with relative ease, it is relatively difficult to solder the shields to filter housing 109 once the shields are positioned within the confines of the housing. In order to achieve this type of soldering operation, it is necessary to apply a high heat source to the exterior of filter housing 109, which can adversely effect the characteristics of the electrical components already positioned on circuit board 100. It is also difficult to control the flow of solder within the confines of filter housing 109, and thus it is not uncommon for one or both sides of the filter to become shorted. Such shorted filters must, of course, be discarded.
Another problem relates to flange 107. The flange is present on each shield in order to guide circuit board 100 into filter housing 109 and to provide a soldering surface parallel to the wall of filter housing 109. The electronic components on the circuit board, however, must be spaced away from the shields by a distance greater than the longitudinal length of flange 107 to allow the shields to be inserted into slots 101 on opposite sides of circuit board 100. The longitudinal length of flange 107, therefore, unnecessarily increases the overall length of the filter. This problem is even more noticeable in six-pole and eight-pole filters, which use multiple shields.
Yet another problem is that the shields, being separate components, increase the total number of components that must be handled during assembly of the overall filter device. This in turn increases manufacturing time and expense.
It would be desirable to provide an electrical signal filter having isolation shields that are easy to handle and solder within the filter housing. No such filter currently exists.
It is an object of the present invention to provide an electrical signal filter that can overcome all the drawbacks associated with the prior art filters discussed above. In accordance with one object of the present invention, an electrical signal filter is provided that includes an elongate lower filter housing member extending along a first longitudinal direction from a first end thereof to an opposed second end thereof, and having an inner surface terminating at first and second upper side surfaces that extend from the first end to the second end. The filter also includes an elongate upper filter housing member extending along the first longitudinal direction from a first end thereof to an opposed second end thereof, and having an inner surface terminating at first and second lower side surfaces that extend from the first end to the second end. The elongate upper filter housing member abuts the elongate lower filter housing member at the first and second lower and upper side surfaces, respectively, along a longitudinal junction. An isolation shield is formed integrally with at least the elongate lower filter housing member and extends inwardly and upwardly from the inner surface thereof in a direction substantially perpendicular to the first longitudinal direction to thereby define first and second internal filter cavities separated by the isolation shield. A first filter section is arranged in the first internal filter cavity and a second filter section, electrically connected to the first filter section, is arranged in the second internal filter cavity. The isolation shield provides magnetic isolation between the first and second filter sections.
In accordance with a preferred embodiment, the isolation shield extends inwardly and upwardly from the inner surface of the elongate lower filter housing member to a position proximate a plane intersecting the first and second upper side surfaces thereof, and a second isolation shield is formed integrally with the elongate upper filter housing member and extends inwardly and downwardly from the inner surface thereof in axial alignment with the isolation shield of the elongate lower filter housing member.
More preferably, the upper surface of the isolation shield of the elongate lower filter housing member has a shape that is complementary to a shape of the lower surface of the second isolation shield of the elongate upper filter housing member, and those surfaces mate with each other to form the appearance of an integral shield.